Effects Of Composition And Structure On Mechanical Properties Of Zr-Co-Al-(Fe) Bulk Metallic Glasses

Bulk metallic glasses （BMGs） have received much attention due to their manyunique properties, for example superior strength, large elastic strain limit （up to2%） andrelatively low Young’s modulus compared to their crystalline counterparts. However,the deformation of BMGs at room temperature is highly localized into shear bands,which usually propagate rapidly across the sample due to strain softening and/orthermal softening, causing catastrophic failure along one dominant shear plane andlimited plastic strain. The factors governing the localization and instability of shearduring the plastic deformation of BMGs are still elusive.In this paper, Zr-Co-Al-（Fe） BMG has been selected. The mechanical propertiesand crystallization behaviors have been investigated systematically. The changes ofstructure and organization of shear bands from generated to extend until sample fracturehave been characterized. Partial replacement of Co with Fe and Co/Al concentrationratio upon the structural, thermodynamic, mechanical and physical properties ofZr-Co-Al-（Fe） series alloys have been studied systematically. And the effects of internalstate on mechanical properties of BMG are discussed too. The results allow thefollowing conclusions to be drawn:①Fully amorphous Zr₅₆Co₂₈Al₁₆rods with a diameter of2mm is fabricated usingwater-cooled copper-mold suction casting. The alloy exhibits high fracture strength ofapproximately2136MPa and a pronounced plastic strain of10.2%. No strain-hardeningbehavior is observed. The plasticity can be attributed to the formation and interaction ofmultiple shear bands during deformation. The activation energies of the glass transition（Eg）, the onset of the crystallization （Ex） and the two stages of the crystallization （Ep1and Ep2） were calculated to be Eg=303.2±13.5, Ex=316.4±37.9, Ep1=336.2±36.2and Ep2=362.0±29.5kJ/mol, respectively. The study of the crystallization behaviorindicates that the phase after the primary crystallization event is mostly composed ofZr₆CoAl₂; a mixture structure of Zr₆CoAl₂, ZrCoAl and B2ZrCo phases is identifiedwhen annealed at823K for100min and heated to923K。However, all samples afterheat-treatments indicate brittle fracture behavior. It can be seen that brittle Zr₆CoAl₂intermetallic compounds may eventually deteriorate the mechanical properties ofmaterials. ②The substitution of Co with Fe element in Zr₅₆Co_28-xAl₁₆Fe_x（x=0,1,2,4,8and14, respectively） was systematically investigated. It is found that the addition of Feimpacts the structural, thermal and mechanical properties remarkably. Enhanced glassystability and glass forming ability （GFA） and compressive plasticity are obtained for x≤4. Higher Fe contents deteriorate the GFA and the mechanical performance. It is furtherrevealed that Fe addition can significantly affect the internal states thus allowing to tunethe yield strength and the plasticity of BMGs. Among the investigated alloys, theZr₅₆Co₂₆Al₁₆Fe₂BMG has good GFA and excellent mechanical properties, e.g. highfracture strength of over2GPa and a pronounced compressive critical fracture strain of15.1%. More Fe concentration （x=8and14） lowers the strength and plasticity due tothe existence of possibly brittle intermetallic phases （for example Al2CoZr6phase）.③A series of Zr₅₆Co_44-xAl_x（x=12,14,16,18and20, respectively） alloys with adiameter of2mm are completely amorphous. With the increasing Al content, the glasstransition temperature （Tg）, the onset crystallization temperature （T x） and the width ofsupercooled liquid region （Δ Tx） are gradually increasing. These alloys indicateenhanced plasticity under room temperature compression tests. Among them, theZr₅₆Co₂₆Al₁₆Fe₂BMG has the best mechanical properties, e.g. high fracture strength ofover2477MPa and a pronounced compressive critical fracture strain of13.3%.④The impact factors on yield strength of amorphous alloys have been studied.The yielding of BMGs is closely related to glass transition, density and molar mass ofmaterials. However, the glass transition temperature and the molar mass govern thespecific strength of materials. The yield strength of Zr₅₆Co_28-xAl₁₆Fe_x（x=0,1,2and4,respectively） increases with increasing Fe content. The trends in this work consistentwith the calculations from theoretical model. With the increasing Co/Al concentrationratio, glass transition temperature （Tg） increases, density decreases, molar massdecreases, thus the specific strength gradually increases. The findings are likely to befavor of alloy design with high specific strength.⑤Fully amorphous Zr₅₆Co₂₈Al₁₆alloys are prepared by suction casting into acopper mold by tuning the casting current. Optimization of the current can induce largeplasticity in an otherwise brittle BMG. It is found that internal state is closely related tothe plasticity of the sample. Further investigation confirms that larger plasticitycorresponds to the internal states with more average FV as revealed by lower density, ahigher enthalpy change, lower hardness and higher Poisson’s ratio. The results provide a clear experimental evidence for the role of FV on the plasticity and have implicationsfor understanding the deformation mechanism of BMGs.